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Published online 1 May 2008
Published in Soil Sci Soc Am J 72:775-785 (2008)
DOI: 10.2136/sssaj2006.0378
© 2008 Soil Science Society of America
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SOIL CHEMISTRY

Organic Amendments Influence Soil Organic Carbon Pools and Rice–Wheat Productivity

Bidisha Majumdera,*, Biswapati Mandalb, P. K. Bandyopadhyayb, A. Gangopadhyaya, P. K. Manib, A. L. Kundub and D. Mazumdarc

a Jadavpur Univ., Kolkata, West Bengal 700 032, India
b Directorate of Research, Bidhan Chandra Krishi Viswavidyalaya, Kalyani, West Bengal 741 235, India
c Dep. of Agril. Statistics, Bidhan Chandra Krishi Viswavidyalaya, Mohanpur, West Bengal 741 252, India

* Corresponding author (bidishamajumder{at}yahoo.co.in).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil organic C (SOC) pools under long-term management practices provide information on C sequestration pathways, soil quality maintenance, and crop productivity. Farmyard manure (FYM), paddy straw (PS), and green manure (GM) along with inorganic fertilizers were used in a 19-yr-old rice (Oryza sativa L.)–wheat (Triticum aestivum L.) cropping system in subtropical India to evaluate their impact on SOC stock, its different pools—total organic C (Ctot); oxidizable organic C (Coc) and its four fractions of very labile (Cfrac1), labile (Cfrac2), less labile (Cfrac3), and nonlabile C (Cfrac4); microbial biomass C (Cmic); and mineralizable C (Cmin). Cropping with only N–P–K fertilization just maintained SOC content, while N–P–K plus organics increased SOC by 24.3% over the control, their relative efficacy being FYM > PS > GM. A minimum of 3.56 Mg C ha–1 yr–1 was required to be added as organic amendments to compensate for SOC loss from cropping. The passive (Cfrac3 + Cfrac4) pool and Cmin constituted about 39 and 11.5%, respectively, of Ctot. Organics contributed toward the passive pool in the order FYM > PS > GM. Most of the pools were significantly (P = 0.005) correlated with each other. Yield and sustainable yield index were strongly related with Cfrac1, Coc, Cmic, and Cmin. Results suggest Cfrac1 as a useful indicator for assessing soil health, and balanced fertilization with FYM as suitable management for sustaining crop productivity of the rice–wheat system.

Abbreviations: BSR, basal soil respiration • Cfrac1, very labile carbon • Cfrac2, labile carbon • Cfrac3, less labile carbon • Cfrac4, nonlabile carbon • Cmic, microbial biomass carbon • Cmin, mineralizable carbon • Coc, oxidizable organic carbon • Ctot, total organic carbon • FYM, farmyard manure • GM, green manure • MQ, microbial quotient • PS, paddy straw • qCO2, respiratory quotient • SOC, soil organic carbon • SYI, sustainable yield index


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Information on organic C stocks in agricultural soils is important because of the effects of SOC on climate change and on crop production. The SOC stock at any time reflects the long-term balance between additions of organic C from different sources and its losses through different pathways. Following the adoption of large-scale intensive cropping, this long-term balance was modified since intensive cropping encourages oxidative losses of C due to continued soil disturbance, while it also leads to a large-scale addition of C to the soil through crop residues. This may cause either a net buildup or a net depletion of SOC stock (Cole et al., 1993; Rasmussen et al., 1980; Kong et al., 2005).

Cropping systems and management practices that ensure greater amounts of crop residue returned to the soil are expected to cause a net buildup of the SOC stock. Identifying such systems or practices is a priority for sustaining crop productivity. To better understand the mechanisms by which C is lost or stabilized in soil, the SOC stock is separated into a labile or actively cycling pool, a slow pool, and a stable or passive, recalcitrant pool with varying residence times (Parton and Rasmussen, 1994). The labile C pool is the fraction of SOC with the most rapid turnover rates. On oxidation, this pool of SOC adds to the loading of CO2 to the atmosphere to accentuate the process of global warming. At the same time, this pool is important from the point of view of crop production. It fuels the soil food web and therefore greatly influences nutrient cycling for maintaining soil quality and its productivity (Chan et al., 2001; Janzen, 1987; Majumder, 2007). This pool is also sensitive to land management changes. The highly recalcitrant or passive pool is, on the other hand, altered only very slowly by microbial activities and hence hardly serves as a good indicator for assessing soil quality and productivity (Weil et al., 2003; Sherrod et al., 2005; Majumder, 2007).

Most of the conventional methods used in SOC determination aim to maximize oxidation of C (Walkley and Black, 1934; Nelson and Sommers, 1982). This approach may not be useful for assessing different management practices regarding their effect on the quality of a system for sustainable crop production, since in many cases it has been observed that total SOC failed to serve as a sensitive indicator for such an assessment (Chan et al., 2001). The adoption of procedures that can preferentially separate the more labile pools might be a useful approach for characterizing SOC and assessing the quality of a system under different management practices. Several techniques based on chemical, physical, and biological principles are used for partitioning labile pools of SOC (McLauchlan and Hobbie, 2004). Some of the commonly used techniques are: oxidation under a gradient of oxidizing conditions (Walkley, 1947; Blair et al., 1995; Chan et al., 2001), floatation with a dense liquid (Gregorich and Janzen, 1996), sieving into different size class separates and their associated C (Six et al., 1998), mineralization under controlled temperature and moisture (Pastor et al., 1993), and direct measurement of the pool size of living soil organisms and microbial biomass (Paul et al., 1999). Some of the important labile pools of SOC currently used as indicators of soil quality are microbial biomass C (Cmic), mineralizable C (Cmin), oxidizable organic C (Coc) fractions, and light-fraction C.

The rice–wheat cropping system occupies about 13.5 million ha in the Indo-Gangetic Plains of South Asia and provides food for 400 million people (Ladha et al., 2003). The crops are grown with adequate amounts of fertilizer and different organics, as available. Several reports, however, have indicated a widespread declining or stagnating trend of yields in the long-term rice–wheat cropping system in Asia. A decrease in SOC has been identified as the major cause for this (Swarup et al., 2000; Yadav et al., 2000; Ladha et al., 2003). To offset such a decrease (in SOC), different organic amendments such as manure (farmyard manure or green manure), compost, and crop residues (particularly rice straw) are commonly recommended. When applied, a part of their C is stabilized into SOC and distributed among different pools. This process is governed by an interplay of factors including climate, substrate biochemistry, C loading, soil, associated precinct, and so on. As such, the different organic amendments are likely to differentially affect the amount of C stabilized and the size and dynamics of SOC pools and ultimately crop productivity. Accordingly, it is hypothesized that the long-term rice–wheat cropping system with balanced fertilization in combination with or without different organic amendments may influence SOC content, the size of its active or labile pools, and hence the soil quality and sustainable productivity of the system.

A number of studies have been done on the changes in labile pools of SOC under different soil management practices, but most of them are incomplete in the sense that they included only one or two or at best three labile pools of SOC. Furthermore, they have been confined to the cooler, temperate regions of the world (Wu et al., 2003; Sherrod et al., 2005). Only a very few such studies have been done so far in tropical and subtropical regions of the world (Rudrappa et al., 2006), although these studies are of special importance in the region since the organic C stock of soils here is inherently low. This low C stock poses a serious threat to soil health and thus long-term sustainable crop production in the region (Mandal, 2005; Sharma et al., 2005). In India, a series of long-term fertility experiments was started, using both organic and inorganic sources of nutrients, during the late 1960s and early 1970s when fertilizer-responsive high-yielding cultivars of crops were introduced. Productivity analysis, nutrient balance, and soil quality studies were made for some of these experiments (Singh et al., 2004; Chaudhury et al., 2005; Mandal, 2005; Sharma et al., 2005), but no attempt was made to study the impact of long-term continuous cropping using balanced fertilizer with or without different organic amendments on the SOC stock and dynamics of its various pools in relation to crop productivity. In the present study, an attempt has been made toward this end using a 19-yr-old experiment with a rice–wheat cropping system on an Inceptisol located in the hot, humid, subtropical region of India with an ultimate aim of identifying a management practice for the system that would improve pools of SOC stock and, at the same time, crop productivity.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site Description
A long-term field experiment with a rice–wheat cropping system on an Inceptisol located in the hot, humid subtropics was established at the University Teaching Farm, Bidhan Chandra Krishi Viswavidyalaya, West Bengal, India (23° N, 89° E, 9.5 m above sea level), in 1986 along the new alluvial soil zone of the state. The experimental site receives an average annual rainfall of approximately 1480 mm and experiences mean annual minimum and maximum temperatures of 12.5 and 36.2°C, respectively. The soil is sandy loam (hyperthermic Aeric Haplaquept according to U.S. Soil Taxonomy), with pH 7.2, and contains 500, 295, and 205 g kg–1 of sand, silt, and clay, respectively. The initial surface soil (0–0.2 m) of the experimental site had total (estimated from an archaic sample) and oxidizable (Walkley and Black, 1934) organic C of 14.2 and 8.8 g kg–1, respectively, CaCO3 equivalent of 1.0 g kg–1, bulk density of 1.2 Mg m–3, and cation exchange capacity of 22.0 cmolc kg–1. Because of high rainfall, a rice-based cropping system predominates in this region.

Treatments and Crop Management
Two crops, rice and wheat, were grown annually with necessary plowing, on average, to a depth of 0.15 to 0.20 m using a power tiller. The experiment was laid out in a randomized block design with the following treatments: (i) control (no N–P–K fertilizers or organics), (ii) inorganic N–P–K fertilizer, (iii) N–P–K + FYM, (iv) N–P–K + PS, (v) N–P–K + GM, and (vi) a fallow where no crop was grown since the initiation of the experiment. Each treatment was replicated four times. Farmyard manure, PS, and GM were used as organics. For green manuring, Sesbania sesban (L.) Merr. was grown in an adjacent field, and the aboveground biomass of the 40- to 45-d-old crop was collected and chopped into small sizes. This GM and well-decomposed FYM were manually spread uniformly on the surface of the specified plots (size 8 by 8 m) at 8.0 and 7.5 Mg ha–1, respectively, on a wet-weight basis and mixed thoroughly with the soil using a power tiller 2 to 5 d before puddling (i.e., breaking down of soil aggregates through tillage under submergence to create a soft layer at the top for easy transplanting of rice seedlings and an impervious layer below for curbing leaching). For using PS as an organic, a good amount of it was collected after threshing, chopped, and kept in a pit with adequate moisture (1:0.8 PS/water) for 4 to 5 mo to partially decompose it, and thereafter it was similarly applied at 10.0 Mg ha–1 on a wet-weight basis and mixed well with the soil using a power tiller 3 wk before puddling.

Native vegetation (shrubs, herbs, grasses, etc.) of the site was allowed to grow freely in the fallow plots without any inputs (fertilizer, water, or plant protection chemicals). During land preparation for transplanting rice, the native vegetation was cut and incorporated into the soil through a shallow plowing (0.10-m depth) so that it was not blown away by the wind. As such, the fallow plots experienced little stresses from cultivation and maintained a good amount of labile SOC and microbial activities. These plots were a part of the original experimental design maintained throughout the experimental period (19 yr) for assessing the relative ability of different treatments for their aggrading–degrading effects on soil quality and also for accommodating any additional treatments of relevance, if required, in the future into this long-term experiment.

The remaining plots were kept flooded with water for a day, then puddled (0.20-m depth) with the power tiller, followed by a laddering for leveling the land. One week thereafter, on average, 30-d-old rice seedlings were transplanted in July with a spacing of 0.20 by 0.20 m. After the rice was harvested in the last week of October, land preparation (plowing and laddering) was done and wheat was sown in the second week of November with a spacing of 0.2 m between rows, and harvested in April. The N–P–K fertilizers, at 120–60–60 (N–P2O5–K2O) for rice and 100–60–40 (N–P2O5–K2O) for wheat as recommended by the State Agricultural Department for those crops on the basis of average soil fertility indices of the region, were applied in the form of urea, single superphosphate, and muriate of potash following standard schedules uniformly throughout the experiment. Their amounts in the N–P–K plus organic treatments were adjusted, however, making allowance for the nutrients contained in the added organics, which were estimated annually following standard methods described by Page et al. (1982). Other recommended practices for raising the crops such as weeding, irrigation, and plant protection measures were followed. All the aboveground biomass for the crops was harvested (leaving 5–7-cm short stalks on the ground) on a whole-plot basis manually by a sickle and removed from the field for mechanical threshing by a paddle thresher. Grain and straw yield for both crops on a whole-plot (8- by 8-m) basis were measured and then expressed in kilograms per hectare.

Crop Residue and Organic-Derived Carbon Inputs into Soil
After harvesting the crops, representative samples of the leftover stubble (the short stalks left after harvest) and roots were collected from each of the four replicated plots of the different treatments for the last 5 yr (since 2000), processed, and analyzed for their C content. Sampling for the roots was done following the method described by Thangaraj and O'Toole (1986). The total biomass yield of stubble, roots, and rhizodeposition were computed as 190, 25, and 150 g kg–1, respectively, for rice and 220, 30, and 126 g kg–1, respectively, for wheat of the total aboveground biomass harvested at maturity for the 38 rice–wheat crops during the 19 yr of the experiment (Bronson et al., 1998). The cumulative crop residue (stubble, roots, and rhizodeposition) C inputs into the soils were then calculated by multiplying the above total biomass amount of stubble, roots, and rhizodeposition by their respective mean C concentrations measured annually during the last 5 yr. The extra C input into the soil through photosynthetic aquatic organisms of the rice field was also estimated following Saito and Watanabe (1978).

The cumulative C inputs into the soil through organics (FYM, GM, and PS) were similarly computed by multiplying the total dry weight of the organics added during the 19-yr period of the experiment with their respective mean C concentrations measured during the last 5 yr. The biochemical composition of the organics was also determined for the last 5 yr following the methods described by Rahn et al. (1999).

Soil Sampling and Analysis
Three representative field-moist soil samples were collected from each of the plots in each replication from 0- to 0.2-, 0.2- to 0.4-, and 0.4- to 0.6-m depths with a bucket auger on the seventh day after rice harvest. They were pooled together to make a composite sample for each depth and replication, then hand crushed, passed through a 2.0-mm sieve, stored at 4°C, and used fresh within 24 h for estimating soil microbial biomass C and mineralizable C. A portion of the field-moist soil samples was oven dried, passed through the same sieve, and used for analysis of different pools of C. Additional triplicate samples were taken from all three depths using a core sampler (0.05 m in diameter, 0.08 m in length) for measuring the bulk density of the soil following the method described by Blake and Hartge (1986).

Total Organic Carbon
The soil samples were oven dried, powdered, and passed through a 2.0-mm sieve, while the organics (FYM, PS, GM, stubble, and roots) were oven dried and finely ground in a mechanical grinder following the methods described by Nelson and Sommers (1982). They were all analyzed for C by a LECO CHN analyzer (Foss Heraeus Elemental Analyzer CHN-O-RAPID, Hanau, Germany). Soil samples were also analyzed for inorganic C titrimetrically by digesting them with dilute HCl following the method of Bundy and Bremner (1972). The Ctot (obtained by Ctot = Leco C – HCl-C), expressed as megagrams per hectare for each of the three (0–0.2-, 0.2–0.4-, and 0.4–0.6-m) depths, was computed by multiplying the Ctot content (g kg–1) by the bulk density (Mg m–3) and depth (m). The amount of C left over, stabilized, and sequestered in the entire 0- to 0.6-m depth was estimated as

Formula 1[1]

Formula 2[2]
where CNPK+org represents C in N–P–K fertilizer plus organic amendments (FYM, GM, or PS), CNPK is the C in the N–P–K treatment at 0- to 0.6-m depth, and Corg represents C applied through FYM, GM, or PS during 19 yr.

Formula 3[3]
where SOCcurrent and SOCinit indicate the SOC stocks in 2004 (current) and at the initiation of the experiment (in 1986). Positive and negative values indicate SOC gains and losses, respectively, for the cropping system. A relationship (linear regression equation) between such changes in SOC (Ctot) (Y) and the total cumulative C inputs (crop residue + organics) to the soils (X) over the years was computed. The critical amounts of C inputs causing a zero change in SOC in soils were calculated (X) from the equation having Y = 0.

Oxidizable Organic Carbon and Its Fractions
The content of Coc and its different fractions in the soil was estimated through the Walkley and Black (1934) method as modified by Chan et al. (2001) using 5, 10, and 20 mL of concentrated (18.0 mol L–1) H2SO4 and K2Cr2O7 solution. This resulted in three acid-aqueous solution ratios of 0.5:1, 1:1, and 2:1 that corresponded to 6.0, 9.0, and 12.0 mol L–1 H2SO4, respectively, and caused the production of different amounts of heat of reaction to bring about oxidation of SOC of different oxidizability. The amounts of Coc thus determined allowed separation of Ctot into the following four fractions of decreasing oxidizability as defined by Chan et al. (2001):

Fraction I (Cfrac1, very labile): organic C oxidizable under 6.0 mol L–1 H2SO4
Fraction II (Cfrac2, labile): the difference in Coc oxidizable under 9.0 mol L–1 and that under 6.0 mol L–1 H2SO4
Fraction III (Cfrac3, less labile): the difference in Coc oxidizable under 12.0 mol L–1 and that under 9.0 mol L–1 H2SO4 (the 12.0 mol L–1 H2SO4 is equivalent to the standard Walkley and Black method)
Fraction IV (Cfrac4, nonlabile): residual organic C after oxidation with 12.0 mol L–1 H2SO4 when compared with Ctot

Mineralizable Carbon
A 25-g portion of field-moist soil samples was wetted to 50% water-filled pore space. It was placed in 1.0-L canning jars along with vials containing 10 mL of 1.0 mol L–1 NaOH to trap the evolved CO2, and incubated for 24 d at 25 ± 2°C. Alkali traps were replaced at 3 and 10 d and finally removed at 24 d. Evolved CO2 was estimated by titrating the excess alkali in the traps with 1.0 mol L–1 HCl to a phenolphthalein endpoint (Anderson, 1982). Basal soil respiration (BSR), an estimate of potential microbial activity, was calculated as the linear rate of respiration during 10 to 24 d of incubation. The total amount of CO2–C evolved during the 24-d incubation was taken as a measure of the potential Cmin of the soil (Franzluebbers and Arshad, 1996).

Microbial Biomass Carbon
Field-moist soil samples (25 g) were fumigated with CHCl3 vapor for 24 h and then extracted with 0.5 mol L–1 K2SO4 (Vance et al., 1987). A second, unfumigated set of samples was also similarly extracted. The difference between the C obtained from the fumigated and unfumigated samples was taken to represent the microbial C flush. The Cmic was calculated using the relationship Cmic = 1/0.41 x C flush (Voroney and Paul, 1984). A microbial quotient (MQ) was calculated as the ratio of Cmic to Ctot. The respiratory quotient or metabolic quotient (qCO2) was calculated as the ratio of Cmin to Cmic and expressed as grams CO2 evolved per day per gram of Cmic.

Sustainable Yield Index and Yield Equivalent
Total crop productivity of the rice–wheat system was calculated through a sustainable yield index and equivalent rice yield using yield data for the 38 rice–wheat crops harvested during the entire 19-yr period of the experiment. This was done to offset annual variations in yield and highlight performance of the treatments during the entire experimental period. The sustainable yield index is defined as

Formula 4[4]
where Formula 4 is the estimated average yield of a practice across the years, {sigma} is its estimated standard deviation, and Ymax is the observed maximum yield in the experiment during the years of cultivation (Singh et al., 1990). Average equivalent rice yield for wheat was calculated as

Formula 4
where unit price is the procurement price fixed by the government for a unit quantity of the respective grain during the harvesting season.

Statistical Analysis
Statistical analysis was performed by the Windows-based SPSS program (Version 10.0, SPSS, Chicago, IL). The SPSS procedure was used for analysis of variance to determine the statistical significance of treatment effects. Duncan's multiple-range test was used to compare treatment means. Simple correlation coefficients and regression equations were also developed to evaluate the relationships among the response variables (equivalent rice yield, sustainable yield index [SYI], Cmin, Cmic, Cfrac1, Coc, etc.) using the same statistical package. The 0.05 probability level is regarded as statistically significant.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Biomass Yield and Quantity and Quality of Organic Residue
On average, grain yield was higher for both crops with N–P–K plus organic amendments than only N–P–K or control treatments (Fig. 1 ). This effect of organics was more pronounced with FYM for rice and with PS for wheat (separate data not presented). Only with the control treatment was there a declining trend in system productivity during the 19-yr period of the experiment. This was true particularly for rice. Of the five tested treatments, the equivalent rice yield and annual C input return to soil were highest with N–P–K + FYM, followed by N–P–K + PS > N–P–K + GM > N–P–K > control treatments (Table 1 ). Sustainable yield index values were also higher with N–P–K plus organics compared with only N–P–K or control treatments.


Figure 1
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  		Fig. 1. Combined (rice and wheat) grain yield of the rice–wheat cropping system under different treatments (1986–2004): NPK, inorganic N–P–K fertilizer; GM, green manure; PS, paddy straw; FYM, farmyard manure.

 

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  		Table 1. Annual return of crop residue C to soil, equivalent rice yield, and sustainable yield index (SYI) under different treatments of the rice–wheat system (mean of 19 yr of experimentation).

 
The quantities of stubble, root biomass, and rhizodeposition (rice and wheat together) were higher in N–P–K plus organics than in N–P–K only or control treatments (Fig. 2 ). On average, C concentrations (on an oven-dry basis) in stubble and roots were 318 ± 21 and 412 ± 33 g kg–1, respectively, for rice and 352 ± 26 and 391 ± 37 g kg–1, respectively, for wheat. The C concentrations for the three organics were 333 ± 51, 420 ± 23, and 415 ± 18 g kg–1 for FYM, PS, and GM, respectively (Table 2 ). The FYM also contained the highest amount of ash (253 ± 36 g kg–1), lignin (175 ± 15 g kg–1), and polyphenol (10.8 ± 3.2 g kg–1); PS had the highest C/N value (97.7) and cellulose content (350 ± 29 g kg–1); GM had the lowest values for all these parameters.


Figure 2
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  		Fig. 2. Combined (rice and wheat) amount of crop residue (stubble, roots, and rhizodeposition) returned to the soil under different treatments in the rice–wheat cropping system (1986–2004): NPK, inorganic N–P–K fertilizer; GM, green manure; PS, paddy straw; FYM, farmyard manure.

 

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  		Table 2. Composition of the organic amendments used.

 
Total Soil Organic Carbon and Carbon Stabilization
Continuous cropping without the use of N–P–K fertilizer or organics (i.e., control) for 19 yr caused a net decrease (13.3%) in Ctot content compared with that under fallow, whereas cropping with the use of N–P–K fertilizers and organics increased the Ctot content by 26.7, 24.1, and 22.0% with FYM, PS, and GM, respectively, over that of the control (Table 3 ). Out of 9.49, 7.98, and 6.31 Mg of C added through FYM, PS, and GM (on a dry-weight basis), respectively, only 6.44, 4.57, and 3.09 Mg, respectively, could be accounted for in the soil in these treatments after 19 yr of cropping (Table 4 ). A significant positive linear relationship (R2 = 0.98, P = 0.001) was observed between changes in SOC (Ctot) and total cumulative C inputs (crop residue plus organics) (Fig. 3 ), which is described by the following equation: SOC sequestered (Mg ha–1) = 0.14 x cumulative C input (Mg ha–1) – 9.19. Figure 3 shows that C inputs of 3.56 Mg ha–1 yr–1 to soils can keep the SOC level unchanged. This is taken as the critical amount of C inputs for maintenance (no depletion) of SOC level.


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  		Table 3. Soil organic C and its fractions at 0- to 0.6-m depth under different treatments after 19 yr of rice–wheat cropping.

 

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  		Table 4. Cumulative amount of C inputs and leftover C in soils at 0- to 0.6-m depth after 19 yr of rice–wheat cropping.

 

Figure 3
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  		Fig. 3. Relationship between cumulative C inputs to soil and change in soil organic C (SOC) stock (critical C input value derived for zero change) under different treatments in the rice–wheat cropping system (error bars represent the standard error of means of change in SOC).

 
Oxidizable Organic Carbon and Its Fractions
Cropping both with and without (control) N–P–K fertilizer caused a decrease (4.5 and 11.3%, respectively) in Coc compared with that in the no-cultivation fallow treatment (Table 3). The N–P–K fertilizer combined with FYM, PS, or GM caused an increase in Coc of as much as 12.4 and 20.9, 10.4 and 18.8, and 8.2 and 16.4% over that in the N–P–K and control treatments, respectively.

The amounts of SOC fractions extracted under a gradient (6, 9, and 12 mol L–1 H2SO4) of oxidizing conditions varied significantly (P = 0.001) among the treatments and depths compared (data not given, pooled for the layers, Table 3). The means of all treatments were in the order Cfrac2 > Cfrac4 ≥ Cfrac1 > Cfrac3, constituting about 32.1, 29.7, 28.9, and 9.3%, respectively, of the Ctot. The N–P–K + FYM and N–P–K + PS treatments had higher values, whereas the control treatment had lower values for almost all the fractions. Such values for the fallow treatment were also higher, for most of the fractions, than that of the control. Again, there was a greater accumulation of all the fractions in the surface compared with the subsurface layers (data pooled for the layers).

Mineralizable Carbon, Soil Respiration, and Microbial Biomass Carbon
Mineralizable C content of the soil varied from 1.15 to 1.92, with a mean value of 1.65 g CO2–C kg–1 soil, constituting about 11.4% of the Ctot (Table 5 ). This was higher (P = 0.001) in N–P–K + FYM (12.4%) and fallow (12.3%) than in the other treatments, with similar trends for BSR. The Cmic content varied from 0.28 to 0.52, with a mean value of 0.43 g C kg–1 soil. The Cmic content of the different treatments were in the following order: N–P–K + FYM (0.52 g C kg–1) ≥ N–P–K + PS (0.48 g C kg–1) ≥ fallow (0.47 g C kg–1) ≥ N–P–K + GM (0.45 g C kg–1) ≥ N–P–K (0.41 g C kg–1) > control (0.28 g C kg–1). The MQ also varied widely between 0.023 and 0.033 kg Cmic kg–1 Ctot, with a mean value of 0.030 kg Cmic kg–1 Ctot (Table 5). Its values were higher in all the fertilized treatments than in the control. On the contrary, the qCO2 was higher under the control and N–P–K than in the N–P–K plus organic treatments, with no variation, however, among the three different organic amendments used.


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  		Table 5. Microbial biomass C (Cmic), mineralizable C (Cmin), basal soil respiration (BSR), microbial quotient (MQ), and respiratory quotient (qCO2) in soils at the 0- to 0.2-m depth under different treatments after 19 yr of rice–wheat cropping.

 
Relationship among Different Organic Carbon Pools and with Crop Productivity
Correlation analyses (data not shown) showed that most of the C pools were significantly correlated (P = 0.005) with each other. Such correlations for Cfrac1 and Coc were comparatively stronger. When the relationship was drawn between SOC pools and crop productivity, data fit well with the linear predictive model for Cmic, Cfrac1, Coc, and Cmin (Table 6 ).


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  		Table 6. Relationship of different soil organic C (SOC) pools with equivalent rice yield and sustainable yield index (SYI) under different treatments after 19 yr of rice–wheat cropping.

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Organic Carbon Depletion and Applied Carbon Stabilization
Cropping during the years caused a net decrease (~13.3% of fallow) in Ctot. Crop cultivation needs plowing, which disturbs the distribution and stability of soil aggregates (Six et al., 2002), thereby exposing the organic C stock in soils to rapid oxidation. This results in depletion of the stock. Many researchers (Jenny and Raychaudhuri, 1960; Swarup et al., 2000), however, have reported a much greater magnitude (30–60%) of this depletion due to cropping in this subtropical part of India. The comparatively lower magnitude of depletion we observed was first due to the soils remaining under a unique aquic (flooded) moisture regime for 3 to 4 mo for the rice crop and second due to the soils being inherently low in organic C content (34.4 Mg ha–1 for the 0–0.6-m depth). These conditions possibly slowed the rate of C oxidation in the soil. Cropping with N–P–K + FYM, PS, or GM brought about a significant increase in Ctot due to an additive effect of N–P–K and the organics and interactions between them. Moreover, a large amount of plant residue (stubble, root biomass, and rhizodeposition) C was left over in the soil owing to increased yield under these treatments (Table 2, Fig. 2). A similar buildup of SOC due to cropping with the application of balanced fertilizer combined with manure (Rudrappa et al., 2006), paddy straw (Verma and Bhagat, 1992), and green manure (Yadav et al., 2000) was also reported from long-term experiments.

It was observed that 67.8, 57.3, and 49.0% of the C applied through FYM, PS, and GM, respectively, was stabilized in the form of SOC (Table 4). This shows that the C applied through FYM was the most resistant while that applied through GM was the least resistant to decomposition. This is because FYM and PS have a higher C/N ratio and lignin and polyphenol contents than GM (Table 2). The higher content of lignin and polyphenol in FYM and PS led to the formation of stable complexes with proteins of plant origin and thus made the FYM and PS C more resistant to decomposition than that of the GM (Tian et al., 1992). Villegas-Pangga et al. (2000) also obtained an inverse relationship between cumulative C release and the C/N ratio in rice straw. The differences in the magnitude of loss among the organic sources (GM 51.0%, PS 42.7%, and FYM 32.2%) were narrowed (63–71%), however, when crop residue C inputs were also included (Table 4). High ash, lignin, and polyphenol contents in the stubble and roots of the rice and wheat crops gave them resistance to decomposition. Results thus suggest that both the quality and quantity of C inputs into the soil have an important role in building up SOC. The total amount of C applied to the soil through all three organics (FYM, PS, and GM) together during the entire period (19 yr) of the experiment was 23.8 Mg C ha–1 (Table 4), of which 14.1 Mg C ha–1, constituting, on average, 58.1% of the added amount, could be accounted for in the form of Ctot in the soil. Furthermore, the combination of organics with N–P–K fertilizer caused a significant increase in crop yield (Fig. 1, Table 1), with an attendant increase in the amount of crop residue C returned to the soil (Fig. 2, Table 1). If this is also taken into consideration, the value of leftover C declines to only 33% of the applied amount (organic plus crop residue C) (Table 4). A strong oxidative force of high temperature (during peak summer months, the temperature can reach 40–45°C) compared with cooler, temperate regions coupled with the disrupting effect of intensive cultivation led to a rapid oxidation of soil organic C in this region.

The existence of a linear relationship (R2 = 0.98, P = 0.001; Fig. 3) between the changes in SOC (Ctot) and the total cumulative C inputs (organics plus crop residue) to the soil indicates that, even after 19 yr of C additions at a reasonably high rate through FYM, PS, and GM (7.5–10.0 Mg ha–1 yr–1 on a wet-weight basis) and crop residues (3.93–4.10 Mg ha–1 yr–1), the soils of the present experiment still have a capacity for storing C and therefore have great potential for further C sequestration. The slope of the curve (Fig. 3) represents the rate of conversion of input C to SOC. This is about 14% of each additional megagram of C input per hectare in this cropping system. We wanted to compare our values with those of others, if any, in subtropical parts of the world but failed to do so since such information in the literature is rare; however, our values were similar to those reported by Rasmussen and Collins (1991) (14.0–21.0%) and Rasmussen and Smiley (1997) (14.8%) from cooler, temperate climatic regions (the United States and Canada) but higher than those obtained by Kong et al. (2005) (7.6%) under Mediterranean-like climatic conditions.

This study also reveals that, to maintain SOC levels (zero change), the critical amount of C input to the soil is 3.56 Mg C ha–1 yr–1 (Fig. 3). This is similar to that reported by Kong et al. (2005) (3.1 Mg ha–1 yr–1) in Davis, CA, under a Mediterranean-like climate. The quantity of crop residue C inputs (3.7 Mg ha–1 yr–1) under the N–P–K treatment almost failed to bring about any significant change (~3.6%) in SOC even after 19 yr of cultivation. In view of this, the observed critical amount of 3.56 Mg C ha–1 yr–1 seems to be a realistic one; however, more such studies need be undertaken under hot, subhumid, tropical climate to substantiate the observed value and look for suitable alternate profitable cropping systems that might provide the amount necessary for upkeep of soil health.

Oxidizable Organic Carbon and Its Fractions
Reports of loss of Coc content in soils due to cropping are many from this subtropical region because of the high temperature (Bhattacharyya et al., 2004, p. 44; Swarup et al., 2000). Carbon supplementation through FYM, PS, and GM increased Coc; however, the percentage increase in Coc over the control was relatively lower than that of Ctot. Accumulation of Coc under organic inputs is likely to be influenced by their composition, C/N ratios, and decomposability or degradability. In spite of having significant differences in these parameters and also in the amounts of C added through the three organic sources (FYM, PS, and GM), there were little differences in the Coc content (62.8, 61.7, and 60.4 Mg C ha–1 soil, respectively) in the soil profile (up to 0.6 m) even after 19 yr of their application.

The amounts of the four fractions (Cfrac1, Cfrac2, Cfrac3, and Cfrac4) of SOC having different lability varied significantly among the treatments and depths (Table 3). These variations were more prominent with Cfrac1 and Cfrac2 than with Cfrac3 and Cfrac4. The current methodology for estimating C fractions would thus be helpful in monitoring even small differences in the effects of different treatments. On average, the higher values of different C fractions under FYM, PS, and GM treatments were ascribed to the increased yield and C returned to the soil. These higher values of Cfrac1 under the N–P–K + FYM and N–P–K + PS treatments particularly were associated with the high content of polysaccharides (cellulose and hemicellulose) in FYM and PS that could lead to the production of higher amounts of labile fraction (Cfrac1) SOC than with GM (Seneviratne, 2000). Many researchers (Halvorson et al., 2002; Dormaar and Pittman, 1980; Rasmussen and Collins, 1991) have shown that there was, in general, a decrease in the values of different pools of C in soils under fallow treatment and the magnitude varied directly with the length of the fallow period. They had fallow periods of varying length between two crops where land was subjected to normal plowing and other forms of perturbation, except during the fallow period. During the fallow period, the land remained bare and was exposed to high temperatures that accelerated the oxidation of SOC. In our experiment, however, the fallow treatment was maintained differently, allowing native vegetation to grow with its subsequent in situ incorporation with minimum tillage and no perturbation for cropping since the initiation of the experiment in 1986. This explained the observed variations in results under the fallow treatments between this study and the others mentioned above.

The first two most easily oxidizable fractions, Cfrac1 and Cfrac2, together constituted about 87.6 and 61.0% of the Coc and Ctot, respectively. Chan et al. (2001), while comparing the effectiveness of different pasture species in maintaining labile pools of SOC, observed a similar proportion (~65%) of Ctot in those two labile fractions in semiarid areas of Australia. They further observed that the other two fractions (Cfrac3 and Cfrac4) accounted for a smaller proportion (~35%) of the Ctot that falls within the range of 30 to 40% assigned to the "passive pool" of SOC used in the Century Model (Parton and Rasmussen, 1994), which is very close to our observations (39% of Ctot). Further, while fractionating C into these four pools in soils from five long-term experiments with different management practices and cropping systems, Majumder (2007) observed strong relationships of Cfract 1 and Cfract 2 with Cmic and Cmin and crop productivity of the systems, although such relationships for Cfract 3 and Cfract 4 were weak. Accordingly, Cfract 1 and Cfract 2 might constitute the "active pool" while Cfract 3 and Cfract 4 might represent the passive pool of SOC (Chan et al., 2001; Majumder, 2007). Comparing our results with those of Chan et al. (2001), it appears that the type of crops cultivated (cereals or legumes and grasses) does not have much effect on the relative proportion of Ctot in labile pools. Climatic conditions (semiarid or subtropical) were also found to have no discernible effect in this regard under the management practices and cropping system tested.

An attempt was made to find out how the application of different organic amendments helped to build up the passive pool (Cfrac3 + Cfrac4) of SOC (Table 4). It was observed that out of 6.44, 4.57, and 3.09 Mg of C inputs in the form of FYM, PS, and GM, respectively, 5.97, 3.86, and 1.08 Mg constituting 92.8, 84.5, and 35.1%, respectively, found their way to the passive pool, leaving the rest to participate actively in soil C cycling. Of the sources, PS and FYM thus contributed more than GM in enriching the passive pool and thus subsequently helped in building up C stocks in the soil. If one considers the highly stable passive pool of SOC with turnover times in the centuries used in the Century Model (Parton and Rasmussen, 1994), however, the stability of the currently built-up passive pool of Cfract 3 and Cfract 4 through added organics during the last 19 yr of the present experiment may not be alike.

Mineralizable Carbon, Soil Respiration, and Microbial Biomass Carbon
The amount of C mineralized (11.4% of Ctot) that we observed (Table 5) within a short span of 24 d is typical in soils of tropical and subtropical regions (Rudrappa et al., 2006). The amount varied under different treatments, being lowest in the control. This is due to the variable amounts of labile organic C fractions in soils under the treatments (Table 3). The higher value of Cmin content in the N–P–K fertilizer with organic amendments and fallow treatments may be attributed to the good supply of labile C substrate in those treatments. The basal soil respiration (Cmin at 10–24 d and BSR) per unit of SOC represents the performance of soil organic C decomposers and also the quality of SOC under decomposition. Like Cmin, the BSR also was found to be higher in the N–P–K with organic supplementation and fallow treatments. As mentioned above, the availability of easily decomposable organic matter and also readily available nutrients in these treatments provided a conducive environment for microbial activity, resulting in a higher rate of respiration (Sayre et al., 2005). The mineralizable C, which provides an early indication of a possible degrading or aggrading effect of different management practices on soil quality (Powlson, 1994), was also lowest in the control treatment. This seems to be related to the unfavorable environment in the control arising out of the depletion of nutrients due to continuous cropping without any fertilization. Balanced fertilization with C supplementation, however, provided a congenial environment for microbial growth and thus caused an increase in soil Cmic over that in the control.

The MQ signifies microbial activity. It varied widely (0.023–0.033) among the treatments (Table 5). This is, however, within the range of 0.01 to 0.05 kg Cmic kg–1 Ctot advocated by Anderson and Domsch (1980). Its higher values in N–P–K plus organics and fallow treatments compared with the control may be attributed to a better nutritional environment in the former treatments. The results also suggest that the organic C under the former treatments is more stable than in the control (Sparling et al., 1992). The lowest value of MQ in the control soils indicates that the capacity of the soil for C cycling has been impaired, signifying a degradation of its quality (Dalal, 1998). A relationship between MQ and Ctot showed that MQ was directly influenced by Ctot (r = 0.80, P = 0.001). Haynes and Tregurtha (1991) also reported a decline in MQ from 0.023 to 0.011 kg Cmic kg–1 Ctot due to a decrease in SOC from 65 to 15 g C kg–1 soil.

The respiratory or metabolic quotient (the ratio of Cmin and Cmic, qCO2) is an indicator of the efficiency of soil microorganisms in processing residue or available soil C. Its value was significantly higher under the control and N–P–K fertilizer than in the fallow and organic-amended soils (Table 5). The higher value of qCO2 in the former treatments suggests a less efficient use of available C by the microbes there, whereas its lower value in the latter (fallow and organic-amended treatments) indicates their higher efficiency in preserving C in soils. There was, however, no difference in qCO2 among the three organic amendments. Higher and lower values of qCO2 associated with fertilizer and organic-amended treatments, respectively, have also been reported by many researchers (Fauci and Dick, 1994; Lupwayi et al., 1998; Rudrappa et al., 2006). Although qCO2 undoubtedly indicates microbial efficiency, several other factors such as soil moisture and qualitative changes within the microbial population (e.g., an increase in the proportion of fungi) may explain the differences in qCO2 within the treatments. For all these active C fractions, the fallow treatment rather than the control treatment compared well with the fertilized and organic-amended treatments, mainly because the fallow treatment had experienced little stress from cultivation and also contained high amounts of labile C pools.

Relationship among Different Organic Carbon Pools and with Crop Productivity
The observed significant correlations among the pools of C in the soil indicate the existence of a dynamic equilibrium among them. This means that depletion or enrichment in one would shift the equilibrium and affect the size of the others. Such relationships, particularly for Cfrac1 and Coc with others, were stronger. This emphasizes the importance of monitoring these two pools for a better understanding of C cycling in the soil. Relationships were also drawn between these pools of C in the soil and the crop productivity of the system. These relationships of both yield and SYI were strong, particularly with Cfrac1, Coc, Cmin, and Cmic (Table 6). They accounted for as much as 63, 63, 30, and 68% of the variability in SYI and 92, 92, 68, and 95% of the variability in rice equivalent yield, respectively. This suggests the importance of these pools of SOC in influencing crop yield, possibly through maintaining better soil quality. Considering the relatively low cost and ease of estimation and also its influence on yield, SYI, and C cycling, Cfrac1 appears to have an edge over the other pools of C in the soil for its inclusion as a good indicator for routine monitoring of soil health in the rice–wheat agroecosystem.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
While continuous rice–wheat cropping without balanced fertilization (N–P–K) caused a significant depletion of SOC, the same with balanced fertilization maintained the level and, in combination with organics, significantly improved it. A major fraction (~67%) of the C supplemented in the system through organic amendments (FYM, PS, and GM) was lost, however, and a small fraction was left to be stabilized into SOC. The relative efficacy of the amendments for SOC building was FYM > PS > GM. To offset the loss and to maintain SOC, a minimum of 3.56 Mg C ha–1 yr–1 needed to be incorporated into the soil in the form of organics. Of the several C pools analyzed, the passive pool constituted about 39% of the Ctot. A higher proportion (70.8%) of C that got stabilized into SOC from C inputs, however, found its way to the passive pool, leaving a smaller part in the active pool. Mineralizable C (24 d) constituted a large (~11.5%) proportion of Ctot in this subtropical region. Of the pools, Cfrac1, Coc, Cmin, and Cmic explained the variability in crop yield to a greater extent. Because of the low cost and ease of estimation and also its influence on yield and SYI, the Cfrac1 fraction of SOC can reasonably be used as a good indicator for assessing soil health and productivity. An overall consideration of the results indicates that, of all the treatments tested, balanced N–P–K fertilization along with an adequate amount of FYM is most suitable for the continuous rice–wheat cropping system in the subtropical Indo-Gangetic Plains of India for improving the SOC stock and sustaining crop productivity.


   ACKNOWLEDGMENTS
 
ACKNOWLEDGMENTS

We are extremely grateful to Dr. L.N. Mandal, former professor of soil science, Bidhan Chandra Krishi Viswavidyalaya, West Bengal, for kindly going through a draft manuscript and offering valuable suggestions for strengthening it. We also express our immense gratitude to Dr. Bill Hardy, Science Editor and Publisher, Communication and Publications Services, IRRI, for editing the manuscript. We are also thankful to the Indian Council of Agricultural Research, New Delhi, for funding the work through the World Bank assisted multi-institutional collaborative National Agricultural Technology Project.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication November 3, 2006.


   REFERENCES
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 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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